IQ, build, performance - basically I'm not aware of anything better in any aspect (except size & weight - but even that's improved over the MkII

Cons:

One niggle - as one of the few who actually liked the joystick on 20D/5D as a way of selecting focus points, I was a little disappointed to see this functionality removed.

OK, its a lot of money. But you get a lot, and for anyone who earns money from photography this should be a sound investment. IQ is fantastic, the best I have seen straight out the camera. The time saved in post-processing alone make this a good buy.

Performance and build have been raved about already. Its all true, and I've not experienced any AF issues.

Battery life - remarkable. I let mine run to 3200 frames before charging (down to last bar, but not flat) - and with LOTS of chimping.

Best feature - 14bit images ... now I have had some time to experiment with the RAW files, I'm beginning to understand that this is a huge step forward. Its not just the out of camera quality - there is so much more scope for post processing.

This camera is much more than a makeover - I'm surprised Canon
did not release it under a new name. It takes digital photography to a new level, and there is simply nothing else in its class.

I don't think there is any dispute as to the image quality of this lens - it gets a less than perfect mark from me because

a) it is ludicrously expensive (I wonder how many Canon would sell if they cut the price by 30%? - I suspect people would switch camera systems to use it)

b) the lens hood is a bit clumsy and the tripod ring can't be removed.

That aside, I love this lens. I rented one on a couple of occasions last summer, and while I was pleased with the results, had some concerns with weight, portablility and hence how much use I would get out of it. Being a rental, I felt obliged to use the Canon carrying case. Big mistake, and totally unnecessary.

I spent much of the next 6 months trying to decide between this and the 400 DO, the latter having the obvious benefits of size and weight. Ultimately I decided it was image quality that mattered, and went for the 500.

In practice the size and weight have not been nearly the issue I feared. Putting the Canon case away in the loft, I discovered this lens will fit in a Lowepro Phototrekker AW (albiet without a body attached) this dispensing with the portability issue. Further, with a bit of experience, this lens is hand-holdable for extended periods - hence my slight annoyance with the inability to remove the tripod ring and shave off a little weight!

To my mind this is the ultimate telephoto selection - the 600 is just plain too expensive, and in my view, just the other side of hand-holdability. Image quality is amazing - with a 1.4x I can detect no loss of quality, and only a slight loss with the 2x. With stacked convertors, I'd equate image quality to say a decent consumer zoom at the long end (provided of course that atmospheric conditions are ideal).